1. Field of the Invention
The invention disclosed herein relates to thermoplastic film-forming resin compositions, and in particular it relates to a multi-layer, heat-sealable film suitable for conventional packaging applications.
2. Brief Description of the Prior Art
Oriented polypropylene film has become a useful and widely accepted packaging film because of its good moisture barrier, stiffness, high strength, and good optical properties. However, films of polypropylene do not exhibit good heat sealing properties, an important consideration in packaging applications. In order to get good heat sealability, it has been the practice to apply various types of coatings to the film. Coatings have conventionally been applied in separate coating operations, such as from emulsions, latices, extrusion coating and so forth, but such operations are costly and require additional handling of the film, and such additional handling can result in damage. Coextrusion has been used successfully to put a heat sealable coating on polypropylene and produces a film with good seat strength, but currently available coextrusion products have a very narrow heat seal range.
U.S. Pat. No. 3,972,964, issued to D. J. Doentremont, discloses a coating composition for heat-shrinkable thermoplastic film comprising blends of polybutene-1 with ethylene-propylene copolymer to provide abuse resistance with reduced tackiness without compromising the orientability of the base film. However, in order to achieve this objective, Doentremont specifies that the coating composition must have a polybutene-1 constitutent in the range of 5%-40% by weight and that it is undesirable to have more than 40% polybutene-1. Such a coating composition, while achieving the objectives of abuse resistance and reduced tackiness, does not impart good heat sealability to the film.
U.S. Defensive Publication No. T-955,009 (R. C. Lansbury and T. G. Heggs, Feb. 1, 1977) addresses the problem of heat sealability directly. Lansbury and Heggs have found that they can improve the sealability of polypropylene film by coextruding the polypropylene substrate with a coating composition comprising a blend of 10%-59% (by weight) ethylene-propylene copolymer with 41%-90% (by weight) of a homo- or copolymer of a C.sub.4 to C.sub.10 alpha-olefin to form a multi-layer film having a polypropylene core with a thin coating of such composition on one or both sides.